The implanting of prosthetic devices such as infusion apparatus or pumps in the body is a relatively recent development. These devices are used to dispense infusate such as hormones or chemicals, for example, into the body to alleviate a particular physiological problem of the patient.
Usually such pumps include a housing made of a material such as titanium that is compatible with the physiological system. The housing defines a cavity divided by a movable wall such as a bellows capsule having one end secured to a housing wall, its opposite end being enclosed. The capsule thus divides the cavity into a pair of chambers whose volumes may be varied reciprocally by extending or compressing the bellows capsule. The chamber inside the bellows capsule is filled with infusate while the chamber defined by the outside of the bellows capsule and the housing wall is filled with a two-phase fluid such as chloroethane which has a significant vapor pressure at body temperatures. As it vaporizes, the fluid compresses the bellows capsule and forces the infusate therein through an outlet conduit leading to the infusion site.
The bellows capsule can be refilled periodically by injecting infusate through a penetrable septum located in the housing wall and leading to the interior of the bellows capsule. In the process of refilling and therefore expanding the bellows capsule, the two-phase fluid is pressurized condensing some of the gas, returning it to its liquid phase. Whereupon the liquid again commences to vaporize and compress the bellows capsule which thereupon dispenses the new charge of infusate. The rate of infusate flow is controlled by a flow restrictor in the outlet from the bellows capsule which is usually simply a length of capillary tubing. Examples of such infusion devices and their usage are described in detail in U.S. Pat. Nos. 3,731,681 and 3,951,147.
In some cases, the flow of infusate from the bellows capsule being compressed by the two-phase fluid is controlled more closely by means of an electrical valve located in the outlet from the bellows capsule. The opening and closing of the valve is controlled by a programmable controller which issues electrical pulses with a selected repetition rate or pulse duration which open and close the valve so that the apparatus dispenses infusate at a programmed rate which may vary with time or in response to changing conditions in the body such as temperature, sugar level, blood pressure or the like. An example of a programmable implantable pump of this general type is described in U.S. Pat. No. 4,077,405.
One problem with the prior infusion apparatus of the type described in the last mentioned patent is the potential for infusate overdose due to valve failure. In other words, the valve controlling the flow of infusate from the reservoir to the infusion site is invariably a normally closed valve which is opened when energized by signals from the controller. Therefore, if the valve fails in its open position (power continuously on), infusate will flow continuously from the collapsing infusate reservoir to the body site rather than at the programmed intervals and durations scheduled by the controller. In this event, there is a risk that the patient will receive an overdose of infusate which might cause a serious health hazard.
Another problem with the prior prosthetic devices of this general type stems from the fact that the flow rate of the infusate from the bellows capsule varies as the pressure of the two-phase fluid acting upon the capsule varies. That fluid pressure, on the other hand, changes as the patient's body temperature changes. Thus if the patient has a fever, the increased temperature causes the two-phase fluid to exert a greater pressure on the bellows capsule than is the case if the patient has a normal temperature thereby resulting in increased infusate flow. A change in atmospheric pressure will likewise change the infusate flow.
As is described in the last mentioned patent, this flow variation caused by temperature change can be compensated for by sensing the pressure of the two-phase fluid by means of a transducer exposed to that pressure and applying the signals from the transducer to the controller to change the repetition rate or pulse duration of the signals applied to the valve. However, the cost of the transducer and its connections to the controller increase the overall cost of the apparatus. Moreover, this means that the apparatus must include another dynamic part which itself can fail in such a way as to cause either an excess of flow or inadequate flow. In this connection, it should be emphasized that it is intended that infusion apparatus of the type with which we are concerned here remain implanted in the human body for a prolonged period, in the order of eight or ten years or more. Therefore it is essential that the apparatus contain as few dynamic parts as possible, those few parts being of unusually sturdy and rugged construction so that they are not prone to failure and when failed, fail in such a manner as to stop the infusion process. The addition of a pressure transducer to monitor the pressure of the two-phase fluid driving the bellows capsule is not consistent with that overall philosophy.
Finally, apparatus of this type sometimes should provide a uniform basal infusion flow to the patient over the long term which flow is supplemented at selected times by relatively short term bolus infusate doses. The last mentioned patent discloses various ways of accomplishing that objective. In one apparatus, infusate flows continuously from a main chamber to provide the basal needs of the patient. The bolus dose is provided by way of a separate flow path having a pulsed electrical valve and extending from the main chamber in parallel with the basal flow path. As discussed in the second mentioned patent, because of the very small basal flow rates required, it is quite difficult to make reliable flow restrictors whose orifices are small enough to provide such small flows. Also the small restriction orifices are prone to blockage.
In another variation of that patented apparatus, the basal and bolus flows are provided via a single valved fluid path, the valve being pulsed differently to achieve the different basal and bolus flow rates. Providing the basal flow in this manner requires that the valve be pulsed substantially continuously. Therefore, the valve consumes a relatively large amount of power requiring relatively frequent replacement of the battery that powers the valve.